1. Field of the Invention
The present invention relates to cooling high density electronic power supplies, and more particularly to distributing the thermal load generated by the power supply.
2. Background Information
During the past decade, there has been a trend to provide smaller and lighter computers, especially personal computers and portable types. The electronics industry has been successful in reducing the size of the electronic components, allowing computers to become smaller and lighter. This trend to smaller and lighter devices has extended naturally to power supplies.
The power supply industry has spent considerable effort to meet this trend to reduce the size and weight of power supplies. One result of this effort is a product category known as xe2x80x9chigh densityxe2x80x9d DC to DC converters. This type of power supply, as is evident from the title, converts a DC input to one or more DC outputs. U.S. Pat. Nos. 5,206,800; 5,075,821; U.S. Pat. No. RE036098 and U.S. Pat. No. 5,291,382 describe such DC to DC converters. During this past decade, however, the majority of power supplies sold have not been the DC to DC converter type. The majority sold have been xe2x80x9coff linexe2x80x9d power supplies, sometimes referred to as xe2x80x9cmains operated.xe2x80x9d This type is powered from 110 or 220 VAC mains, or xe2x80x9clinexe2x80x9d voltages, and outputs one or more DC voltages, and is typified by those sold for personal computers.
With the growth of computer networking, typified by Internet applications and wireless communications, there has been increasing demand for power supplies of greater performance in the same or smaller space. Faster, denser chips and increased functions create a need for more power in a smaller package, i.e., reducing the size of the power supply without reducing its power rating. A major concern of power supply manufacturers is how to remove the heat from these ever smaller supplies. As is well known in the art, higher temperatures reduce lifetimes and can adversely affect performance of electronic components.
This heat removal is the principal barrier to reducing power supply size. In computer/communications products, the power semiconductors remain the principal heat dissipators (heat generators). These semiconductors typically have specified maximum operating and storage junction temperatures above which the devices may be damaged and/or performances of the semiconductors reduced. Lower junction temperatures are usually specified or calculated in practical designs that will ensure proper operation, and the difference from the maximums can be considered as a safety margin for operating the devices. The heat from the semiconductors in these products is ultimately conducted (convected and, to a lesser degree radiated) and absorbed into the some ambient heat sink, e.g. the outside air. How that is done has a significant effect on the size of the power supply. An article by the present inventor, entitled xe2x80x9cCooling a High Density DC-DC Converter Impacts Performance and Reliability,xe2x80x9d published in PCIM, November 1999, describes the basic thermal issues involved in DC converters and is relevant as well to mains operated power supplies. This article is hereby incorporated herein by reference. The rule is that as the heat density (heat generated in a given volume) increases, heat removal techniques must be concurrently improved and monitored.
In many air cooled power supplies, the transfer of heat from the dissipative elements to the moving air is inefficient, with some of the exhausting air hot and some cool. Often, the distribution and circulation of the cooling air is relatively unsophisticated, with minimum effort given to the dynamics of heat transfer. U.S. Pat. Nos. 6,046,921 and 6,081,423 address some of the thermal management issues discussed in the above referenced article. In each of these patents, power supplies or DC to DC modules are arranged with an air flow axis established along a heat sink to cool the electronics. However, not all the considerations of the article are addressed by these patents.
It is an object of this invention to establish a heat removal configuration which provides a mechanism to establish the lowest thermal resistance from the highly dissipative power semiconductors to the ultimate cooling mediumxe2x80x94usually the outside air.
It is another object of this invention to create a mechanism for balancing the thermal load within the unit being cooled to make best use of air flow.
It is a further object of this invention to incorporate a three dimensional alignment of thermal elements that accommodates mechanical system tolerances while maintaining efficient thermal transfer.
Yet another object of the present invention is to spread evenly the heat generated in dissipative semiconductors so as to minimize thermal gradients.
Still another object of the present invention is to position the dissipative power semiconductors relative to their heat sensitivity and the air flow to provide substantially the same thermal safety factor for those power semiconductors.
It is still a further object of this invention to establish a means whereby the thermal and mechanical design allows convenient determination of the operating temperatures of the dissipative power semiconductors.
The above objects and other advantages are met by the present invention by a power supply system and heat removal process incorporating an integral heat distribution feature that facilitates reducing the size of air flow cooled electronic power supplies.
The present invention is based on and assimilates the following parameters: the intended electrical and mechanical power supply specifications, the available air flow characteristics, the electrical and mechanical specifications of the principal heat generating components, e.g. the power semiconductors, the environment or ambient temperature and the temperatures where the power semiconductors become vulnerable as failure-rates become excessive, and the predictability of that vulnerability.
A formal electrical and mechanical design, coupled with empirical observations, establishes the needed cooling requirements of any particular design. The cooling requirements are met, in part, by drawing in outside air and directing that air flow for the maximum heat removal from the power semiconductors and other dissipative components before exhausting the air.
Those components having the highest heat vulnerability, and therefore the highest cooling needs, are positioned to receive the coolest, most turbulent air flow, usually at the air inlet port. The remaining power semiconductors and other dissipative components are positioned along the air flow path to receive warmer, less turbulent air flow inversely in proportion to their heat vulnerability. That is: the more heat vulnerable the components, the cooler the air and more turbulent the air flow.
Those skilled in the art are able to assign relative heat vulnerability safety margins to the different dissipative components and their packages and appropriately position the components along the air flow path.
An aspect of the present invention is to position the power semiconductors and other dissipative components along the air flow path, thereby constituting a thermal distribution system in which all the dissipative components exhibit substantially equal thermal safety margins.
In a preferred embodiment, the heat sensitive power semiconductors are mounted to multiple low thermal resistance metallic bars that are oriented parallel to the air flow path. The bars attach to the ceiling of a metal enclosure and conduct heat from the power semiconductors to the enclosure which further distributes heat. The bars extend from the ceiling to the printed circuit board (PCB) on which are mounted the power semiconductors in a manner to form air flow channels.
The resulting heat transfer to moving air is related, inter alia, to the combined surface areas of the bars and the enclosure inner surface. The enclosure ceiling thermally links the bars to balance and to prevent excessive heating in any particular bar during certain operating modes.
In another preferred embodiment, the multiple metallic bars extend not to an enclosure ceiling but to a finned heat sink structure below the ceiling of the enclosure. In this arrangement, the finned structure is the principal mechanism for transferring heat to the moving air.